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Seminal Science: How Many Seeds Do Different Fruits Produce?

A palatable plant biology activity from Science Buddies



George Retseck

Key concepts
Plants
Fruits
Seeds
Flowers
Biology

Introduction
Do you like your strawberry jelly with or without the seeds? Are you glad to have a seed-free watermelon, or do you enjoy spitting the seeds into the garden? You might not like finding seeds in your fruit, but fruit is a plant's tool for dispersing seeds to create offspring. In this activity you will investigate how many seeds can be dispersed for each type of fruit. Based on the number of seeds they produce, how productive do you think some of your favorite fruits are?

Background
Many plants grow fruit to enclose and protect their seeds, which need to spread out to grow new plants. Animals love to eat sweet, juicy fruit. This approach would seem like a poor way for plants to protect their seeds, so why would making fruit that is tasty be beneficial? When an animal eats fruit the fleshy part is digested. The seeds, however, pass without harm through the digestive system and are spread by the animal when it excretes (poops). In this way, they are deposited farther from the original plant (along with a little bit of fresh fertilizer) and can grow into a new plant. This is called seed dispersal, and it is just one strategy that plants use to spread seeds over a wide area and make more plants.

You might think that all fruit-bearing plants would pack as many seeds as possible into each fruit to maximize the number of new plants that will grow. But, in fact, different plants have different strategies for seed production and dispersal. Some fruits produce many, many seeds to make sure that at least some will grow, even if most fail. Other fruits put all of their resources into producing and protecting one very large seed.

Materials
•     Different types of fruits: Try to include a pepper, tomato and apple as well as a squash or cucumber (yes, all of these are technically considered the "fruits" of their plants)
•     Knife
•     Cutting board
•     Paper towels

Preparation
•     Go to the grocery store and pick out different kinds of fruit. Don't just stick to traditional fruits, try some new ones as well. Some produce you might think are vegetables are really fruit! Try to include at least one pepper, tomato and apple, along with a squash or cucumber. Avoid seedless varieties.
•     Tip: Bananas do have seeds, but they are very tiny, appearing as little black spots in the center of a banana slice. You can try to count them, but it is not recommended!
•     Tip: If you dissect a pepper, be sure to wash your hands before you touch your eyes after handling the seeds. Pepper seeds can be spicy and cause a burning sensation! Use a mild pepper variety, such as a bell pepper, if you are very sensitive.
•     You may need an adult to help you when cutting the fruit open.

Procedure
•     Begin to dissect your first fruit, removing the seeds and placing them on a paper towel. In the fruit, are the seeds arranged in a certain pattern?
•     When you are done removing the seeds, count the number of seeds on the paper towel. How many seeds were in the fruit?
•     Tip: If you are dissecting a cucumber or squash, instead of removing the seeds you can try cutting the fruit lengthwise, counting the rows of seeds, and then slicing the fruit the other way to determine how many seeds are in one row. Multiply these two numbers together to get a good approximation of the total number of seeds.
•     One at a time, continue to dissect each fruit, place the seeds on a paper towel, then count them. Be sure to keep the seeds from different fruits separated.
•     How many seeds are in each fruit? Which held the most seeds? The least? Did similar types of fruit produce similar numbers of seeds?
•     How do seeds from different types of fruit look similar or different? In each fruit, were there similar patterns in which the seeds were arranged?
•     Extra: Try this activity again but use multiple fruit of each type, such as multiple peppers, tomatoes, cucumbers and squash. Does the same type of fruit always hold a similar number of seeds, or does the amount vary a lot?
•     Extra: Is fruit size related to seed quantity? Repeat this activity but this time use a ruler to measure each fruit before you count their seeds to see if larger fruits tend to produce more seeds than smaller ones. (You can also use a scale to weigh each fruit as an alternative way to measure fruit size.) Do larger fruits make more seeds?
•     Extra: Are seedless fruit varieties really seedless? Dissect several different varieties of seedless fruits and look for seeds. Are "seedless" fruit varieties completely seedless, or simply have fewer seeds than normal? What is the decreased seed productivity of seedless varieties compared with normal varieties on a fruit-to-fruit comparison basis?

Observations and results
Did some types of fruit clearly have more seeds than others? Did the cucumbers, squash, tomato and pepper have a lot of seeds, easily over 100 each? Did the apple only have a few seeds, no more than 10?

Fruits are divided into three general groups, with the "simple fruits" group making up the majority we encounter. They're formed from one ovary in one of the plant's flowers. As the ovary turns into fruit, different ovary parts become different fruit parts; when fertilized, small structures called ovules become the fruit's seeds—and more fertilized ovules means more seeds! The other two fruit groups are more complex. In "aggregate fruit"—such as raspberries—multiple ovaries fuse on a single flower. In the third group, called "multiple fruit," many ovaries and flowers unite. A pineapple is a good example of a "multiple fruit."

Cucumbers, melons and squash are simple fruits (they are part of a fruit type called pepo, which are berries) with a firm rind and softer, watery interior. And, as you probably saw, these fruits make many seeds! A zucchini or cucumber can easily have a couple hundred neatly patterned seeds.

Tomatoes, grapes, kiwifruit and peppers are also simple fruits (technically true berries) with fleshier walls and usually very fluid insides—think of how watery a ripe tomato is! Some, like tomatoes and peppers, can have a couple hundred seeds, whereas others, like kiwifruit, can have several hundred! Citrus fruits are berries (a type called hesperidium), too, with leathery rinds and usually only a few seeds.

Similarly, apples and pears also only have a few seeds (10 at most) but are not berries—they belong to a different fruit type, known as pomes, which have some fruit flesh not made from the flower's ovary, but rather from plant tissue near the ovary, which is the same for strawberries.

Cleanup
Dispose of the seeds from your fruit or, if you're motivated and curious, look into how you could grow plants from your seeds. You can eat the rest of the fruit or save it for a tasty, healthy snack later!

More to explore
Plant Structures: Fruit from Colorado State University Extension
How do seedless fruits arise and how are they propagated? from Scientific American
Cache Crop: Rodents May Have Replaced Extinct Megafauna as Seed Dispersers [Video] from Scientific American
Lab 5: Fruits and Flowers from Kellogg Community College
Seed Saving Tips from West Virginia University Extension Service
How Many Seeds Do Different Types of Fruit Produce? from Science Buddies

 

This activity brought to you in partnership with Science Buddies
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